Also known as a one handed quick clamp (oo-er missus). Something like this
https://www.eurocarparts.com/ecp/p/c...CABEgJzVvD_BwE
Though others are available.
You don't need to loosen the bleed nipple but bear in mind that when pushing the piston (gently) in, the fluid will return to the reservoir so loosen the cap of this and make sure it doesn't overflow from here... ( if needed I will draw some out of here using an old turkey baster type thing).
Hope that helps.